326 CLEAVAGE (SEGMENTATION) AND BLASTULATION 



ever, the prototherian mammals are aberrant, highly specialized animals, whose 

 general anatomy and embryology delineates a group quite distinct from the 

 higher mammals. The duckbill or Platypus (Ornithorhynchus) is found only 

 in Australia. The other species belonging to this group is the spiny anteater 

 Echidna aculeata found in New Guinea, Tasmania, and Australia. The duck- 

 bill lays from one to three heavily yolk-laden eggs in an underground chamber 

 on a nest of weeds and grasses. The eggs have a leathery shell. The young are 

 hatched naked, and the mother holds them against her abdomen with her 

 tail, where they feed upon a milk-like substance which exudes from the milk 

 glands by means of pore-like openings. The Echidna lays two white, leathery 

 eggs about the size of the eggs of a sparrow which she places in a temporary 

 pouch or fold of skin on the ventral abdominal wall. They feed similarly to 

 the duckbill young. 



The early cleavages of Echidna and Ornithorhynchus follow different cleav- 

 age patterns. (See Flynn and Hill, '39, '42.) The cleavage planes of the 

 Platypus are more regular and symmetrical and resemble to a degree the 

 pattern of early cleavage in teleost fishes (fig. 160A), whereas the early 

 cleavage planes in Echidna simulate to some degree those found in reptiles 

 (fig. 160B). In both species cleavage is meroblastic. 



In Echidna the cleavage furrows cut almost all the way through the proto- 

 plasmic disc (fig. 161E). The second cleavage in this species is at right angles 

 to the first, and divides the blastodisc into two larger and two smaller cells 

 (fig. 161A). The third cleavage furrows tend to parallel the first furrow, 

 forming eight cells (fig. 161B), while the fourth cleavages run parallel to 

 the second furrow, and 16 cells are formed (fig. 161C). The fifth cleavages 

 lack the constancy of the first four sets although they continue to be syn- 

 chronous; they result in the formation of 32 cells (fig. 161D). 



In transverse section, the cells of the 32-celI blastoderm appear as rounded 

 masses, each cell in its upper portion being free from the surrounding cells 

 but in its lower extremity intimately attached to the yolk substance (fig. 16 IF). 

 Another feature of the early cleavages in Echidna is the tendency of the cells 

 to separate from each other; wide spaces consequently appear between the 

 blastomeres (fig. 161G). This tendency toward independence and isolationism 

 of the early blastomeres is characteristic of the higher mammals, as previously 

 observed. After the 32-cell stage, synchronization is lost and cleavage becomes 

 very irregular. A central mass of blastodermic cells eventually is formed, 

 surrounded by marginal cells, known as vitellocytes (fig. 175A). 



As cleavage and development proceeds, the central blastomeres become 

 free from the underlying yolk, expand, and form a layer about two cells in 

 thickness (fig. 175B). The vitellocytes around the periphery of the blastoderm 

 eventually fuse to form a syncytium or multinucleated cytoplasmic mass inti- 

 mately associated with the yolk (fig. 175B, C). This marginal mass of syncytial 

 tissue forms the marginal periblast. Within the central portion of the blasto- 



